Within the IP (Internet Protocol) Multimedia Subsystem (IMS) as defined by 3rd Generation Partnership Project (3GPP) Session Initiation Protocol (SIP) defined by Internet Engineering Task Force (IETF) is used for controlling communication. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet multimedia conferences, Internet telephone calls, and multimedia distribution. Members in a session can communicate via multicast or via a mesh of unicast relations, or a combination of these. Diameter protocol has been defined by IETF and is intended to provide an Authentication, Authorization and Accounting (AAA) framework for applications such as network access or IP mobility.
Before being able to establish of a communication session, a user has to register to the IMS network. The basic IMS registration procedure is described in subclause 5.2.2.3 of 3GPP specification TS 23.228.
Different types network entities and functions exist in the IMS network. Call Session Control Functions (CSCF) implement a session control function in SIP layer. The CSCF can act as Proxy CSCF (P-CSCF), Serving CSCF (S-CSCF) or Interrogating CSCF (I-CSCF). The P-CSCF is the first contact point for the User Equipment (UE) within the IMS; the S-CSCF actually handles the session states in the network; the I-CSCF is mainly the contact point within an operator's network for all IMS connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator's service area.
The functions performed by the I-CSCF are, for example, assigning an S-CSCF to a user performing SIP registration and routing SIP requests received from another network towards the S-CSCF. The S-CSCF performs the session control services for the UE. It maintains a session state as needed by the network operator for support of the services and may be acting as Registrar, i.e. it accepts registration requests and makes its information available through the location server (e.g. HSS). The S-CSCF is the central point to users that are hosted by this S-CSCF. The S-CSCF provides services to registered and unregistered users when it is assigned to these users. This assignment is stored in the Home Subscriber Server (HSS). An example of a service that is provided by the IMS network for unregistered users is voice mail. The voice mail service enables callers to leave a voice message for a callee (to whom the call was made) who was not reachable for some reason, e.g. not being registered to the network.
The HSS is the master database for a given user. It is the entity containing the subscription-related information to support the network entities actually handling calls/sessions. As an example, the HSS provides support to the call control servers (CSCFs) in order to complete the routing/roaming procedures by solving authentication, authorisation, naming/addressing resolution, location dependencies, etc.                The HSS is responsible for holding the following user related information:        User Identification, Numbering and addressing information;        User Security information: Network access control information for authentication and authorization;        User Location information at inter-system level: the HSS supports the user registration, and stores inter-system location information, etc.;        User profile information.        
Mechanisms at the network element layer like cluster solution may be used to improve the reliability of the S-CSCF. Failover mechanisms may be introduced also at the network layer to ensure the availability of the network service. Therefore S-CSCF re-assignment shall be possible if the assigned S-CSCF is not available.
Currently, for unregistered users, the assignment of the S-CSCF will happen when the first SIP request for an unregistered user arrives at an S-CSCF. This S-CSCF will then treat the user as an unregistered user and try to download the user profile from the HSS using Server-Assignment-Request (SAR). SAR request is a Diameter command message that a Diameter multimedia client sends to a Diameter multimedia server to request the server to store the name of the server (the S-CSCF) that is currently serving the user. The interface between the S-CSCF and the HSS is called Cx interface. If no S-CSCF is assigned to this user at this moment, the HSS will then assign an S-CSCF to this user and provide the user profile to the S-CSCF using Diameter Server-Assignment-Answer (SAA) response over Cx interface.
However, the S-CSCF re-assignment is not possible for unregistered users. If an S-CSCF was already assigned to an unregistered user, the HSS will reject the Cx-SAR request received from another S-CSCF. Thereby, when the currently assigned S-CSCF is not available for an unregistered user, temporarily or permanently, no service can be provided for this unregistered user. However, some services like voice mail box shall also be provided to unregistered users.
Currently it is possible to re-assign an S-CSCF to the IMS user when this user tries to register with the IMS via a SIP REGISTER request sent to the S-CSCF. In this case the registration status of the user is changed and therefore this is not a solution for an unregistered user. In addition there are IMS users who do not register with the IMS at all, e.g. the so-called Public Service Identity (PSI), which represents a service identity.
Further problem in the current IMS is that an I-CSCF can only select an S-CSCF for an IMS user when it receives a non-REGISTER request (e.g SIP INVITE request to initiate a session) and when no S-CSCF is currently assigned to the user. In this case the HSS returns the Server Capabilities of the user to the I-CSCF. The I-CSCF will then select an S-CSCF for the user according to the capabilities, from a list of S-CSCFs stored at the I-CSCF.
However, if an S-CSCF has already been assigned to the user, the HSS returns an S-CSCF address to the I-CSCF using Diameter Location-Info-Answer message (LIA) response. In this case no S-CSCF re-selection is allowed for an I-CSCF. Therefore, when the currently assigned S-CSCF for the user is not available, the I-CSCF cannot re-select another S-CSCF for the user but the I-CSCF must reject the request. Even if the I-CSCF would try to re-select an S-CSCF for the IMS user, the HSS rejects the re-assignment.
S-CSCF assignment to an unregistered user could be removed via administrative means, after which the I-CSCF selects a new S-CSCF for the user and the selected S-CSCF may download the user profile from the HSS. However, this will usually be done for all users assigned with this S-CSCF and may thereby introduce additional load burst at the HSS, which could lead to an overload situation at the HSS. Furthermore the administrative removal of the S-CSCF assignment will usually involve manual work by the service provider and cause additional costs.
The object of the invention is to overcome the above problems.